A member wrote in to the advisory service recently asking for advice on early season brassicas. This farmer has been growing brassicas under row cover (agribon?) and had all of her transplants eaten by flea beetles 2 weeks ago. We’ve put her in touch with an advisor, but curious if anyone has other thoughts/recommendations/advice for her?
Those buggers!
Flea beetles are voracious when the weather is hot and dry and sunny, which it has been for the past few weeks (at least here around London). I’m surprised that the flea beetles did that much damage to her transplants since they were under a row cover. Although I think some people just lay the cover and weigh it down and expect that it’s enough to keep the bugs out. You actually have to bury the sides of the row cover as soon as you transplant to keep the flea beetles out because they will slink under the cover and have a field day under there. And you only lift it to weed or harvest and replace the cover ASAP.
Personally, I find that they do more damage to my direct seeded crops (with the exception of chinese cabbage which they love so I even use it as a trap crop sometimes) like arugula, bok choi, radishes, etc. because they like the really young and tender leaves. I don’t have any of my other brassica transplants under row covers because usually they are strong enough to survive what little damage they do to it.
Last thing I should mention is that brassicas don’t like anything thicker than a P12 or the insect netting in the summer. The thicker stuff raises the temperature too high under the cover. However P12 is also pretty flimsy so… 
I find it important to bury the edges of the row cover & do adequate soil prep before hand so once the plants germinate they can grow quickly - having the necessary water for quick growth can help the plants outgrow flea beetle damage.
A constant challenge.
In the past we have used diatomaceous earth as we plant and then cover and make sure seal the edges. We tend to have only light flea beetle pressure and our brassicas outgrow the beetle. It could be that the heat from last year really made the difference - the only real problem was when we planted right beside a mature brassic section and the beetles just walked over and started munching. Could be that the area you planted into was brassica before and had a high population.
We use crop rotation, monitor, water, use sticky traps, use DE (all in order) and use homemade plan macerations in oil as a last resort. No more row covers for brassicae:)
Diatomaceous Earth I had seen others reference in this form like that, sorry:) You could also use humectable dolomitic lime but it’s stronger and a bit of an overkill even if more local and cost effective.
Further to what Manorun is saying, certain weeds are also in the brassica family. We found that even with insect netting (we have swede midge in the area) the flea beetles were getting into our crops. Seemingly they overwintered on our mustard weeds. Akin to not rotating crops properly, but on a smaller scale.
Finally, I will add that in our experience agribon (row cover) does not keep flea beetles out (even with very well buried edges).
That being said, even our toughest flea beetle infestations were never to the point where we had a crop loss, so we may be missing something. Knowing a bit more about their specific case might help, including whether they are growing in newly broken up soil, or whether the plants were stressed before transplanting.
I believe flea beetles overwinter in plant residue and at field margins, therefore it can be beneficial to coax them out early with a juicy trap crop, then set up your netting over your transplants. Stressed plants always fall victim first, so make sure transplants are healthy and vigorous. It should also be said that the flea beetles thrive in hot and dry weather, so last year was a killer year, hopefully this year will be a more balanced one.